Given a Poco::LogStream binded to a Poco::Logger, and Poco::Logger is made up of a splitter channel which itself has a bunch of channels, what is going to happens if in a multi thread environnment during a << option to the LogStream, the splitter channel is modified? Is this safe?

This is save, as all SplitterChannel methods are guarded by a mutex.
Note that LogStream uses an internal buffer, and only if you write a newline to the LogStream, the contents of the buffer are passed on to the underlying Logger.